The present invention relates generally to electrical circuits and, more particularly, to a low power variable gain amplifier.
Variable gain amplifiers (VGAs) are useful in many applications. VGAs, for example, can be utilized in communications devices (e.g., direct conversion receivers, such as cordless and cellular phones), memory storage devices (e.g., hard disk drives, CD ROM drives, etc.) as well as other electronics, including global positioning (GPS) receivers, wireless local area networks and the like. In particular, VGAs are used in the various parts of such devices, for example, in the radio frequency (RF) input stage, intermediate frequency (IF) and low frequency or baseband circuits of these devices. Because it has variable gain, a VGA can provide a constant output for an input that varies according to changing operating parameters for a particular application.
Various solutions have been proposed to provide variable gain amplification. FIG. 1 illustrates one example of a traditional VGA 10. The VGA 10 includes an input transconductance stage 12, followed by a current steering circuit 14. The circuit 10 depicted in FIG. 1 achieves variable amplification by attenuating the current that flows into the load resistors 16 using a control voltage Vcntl. While this type of circuit arrangement is used in receivers, it is inherently unsuitable for receiver applications, because it has a constant input linearity (set by the input transconductor 12), rather than a fixed output saturation point. In particular, the transconductor 12 requires a sufficiently large input linearity to accommodate the largest input signal that is possible in the system. This places a restriction on the product of the degeneration resistance and the current I. It is possible to make the VGA 10 a low power circuit by lowering I and increasing the resistors 18 of the transconductance stage 12. However, the load resistance 16 is set at the product of the peak gain of the VGA and the degeneration resistance 18. Therefore, as the resistance of resistors 18 is increased, the output resistors 16 must be similarly increased. Due to bandwidth restrictions there is typically a maximum limit on the usable load resistance 16 in the circuit 10. Therefore the maximum value of degeneration resistance of the resistors 18 is limited, in practice. This in turn sets the minimum current that must be dissipated in the circuit to achieve required linearity performance. For typical levels of gain that are required in receiver applications, this current is usually unfavorable.
A second type of traditional VGA circuit 30 is shown in FIG. 2. In this approach, the degeneration resistor 18 of FIG. 1 is replaced by a variable resistor, such as implemented using a MOSFET 32. The gain is varied in this circuit by varying the degeneration resistor. A larger resistance provides a smaller gain and vice versa. This circuit 30 has a constant output saturation level, which is set by the product of the current I and load resistors 34 for all values of the degeneration resistance. A problem with this circuit is that for the largest level of the input (and hence the lowest gain), the MOSFET 32 must be biased such that its drain-to-source impedance is high. This corresponds to a low Vgs-Vth (gate-to-source voltage minus the threshold voltage) of the MOSFET 32. Under this condition, the effective channel resistance is highly non-linear and depends strongly on the values of the drain and source voltages. Thus the amplifier is at its most non-linear behavior for the largest input, which is not a desirable property for a VGA.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the present invention provides a variable gain amplifier that includes an input stage that receives an input signal and converts the input signal into a corresponding intermediate signal. An output stage provides an output signal based on the intermediate signal and a gain control signal, with feedback signal being provided to the input stage as a function of the gain control signal, so that the intermediate signal varies as a function of the input signal and the feedback signal.
According to another aspect of the present invention, an internal gain control signal generator can be employed to convert an external gain control signal, such as from associated digital circuitry, to an appropriate gain control signal that is applied to the amplifier such that the gain of the amplifier varies in a linear-in-dB manner with respect to the external gain control signal.
Another aspect of the present invention provides a method for providing variable gain amplification in a variable gain amplifier. The method includes receiving an input signal at an input stage and converting the input signal into an intermediate signal. Different parts of the intermediate signal are provided to an output of the variable gain amplifier and to feedback circuitry based on a gain control signal. A feedback signal also is provided to the input stage based on the part of the intermediate signal directed to the feedback circuitry.
The following description and the annexed drawings set forth in certain illustrative aspects of the invention. These aspects are indicative, however, of a few ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.